This invention relates generally to microwave lens antennas and, more particularly, to a bootlace lens antenna which reduces quadratic phase errors by use of orthogonal independent delay lines for each bootlace element.
Many Naval applications in electronic warfare and wide-angle surveillance require a microwave antenna the response pattern of which can display a 360.degree. azimuth and at least 90.degree. of elevation. It can be analytically demonstrated that three dimensional (3-D) multibeam microwave bootlace lens antennas cannot be designed to achieve this hemispherical coverage due to aperture limitations. Furthermore, such bootlace lens antennas cannot provide wide angle coverage without incurring large phase errors due to their inability to accurately focus the energy for most beams in the coverage region.
In general, five perfect focal points are possible with a single layered 3-D bootlace lens antenna. That is, only five of the multiplicity of beams over the coverage region will be well focused. The remaining beams will exhibit significant defocusing and aberrational effects due to the generation of large phase errors.
Utilization of a multiple lens system provides additional degrees of freedom which affords a mechanism by which the aberration effects can be reduced. Such an antenna is described in U.S. patent application Ser. No. 06-350,796, filed Feb. 22, 1982 by Pasquale A. Valentino et al, now U.S. Pat. No. 4,458,249 issued July 3, 1984.
This antenna, however, appears to be more complex and lossy.
An alternate design is the Luneberg Lens which is a spherical lens the refraction index of which varies as a function of the radial distance from the center of the sphere. Such a lens is capable of hemispheric coverage because of the property that a feed source placed adjacent any surface point produces a collimated wavefront on the other side of the sphere which travels in the direction of the line from the feed point through the center of the sphere. However, not only is a sphere having a radially variable index of refraction difficult and expensive to construct, but also it is considerably difficult to controllably scan a feed source about the spherical surface to provide hemispheric coverage.
Prior art has demonstrated that instead of constraining the lens design via a set of equations describing perfect foci, one can design a lens based on a minimum RMS error criterion which imposes the constraint that the lens configuration be a figure of revolution. An overall reduction in the focusing error of the lens is then achieved. Analysis of the phase error structure of a bootlace lens antenna designed by this technique, however, has demonstrated that the phase errors in the principal axial planes parallel to the plane of scan (coplanar) and perpendicular to the plane of scan (crossplanar) contain significant quadratic components in opposite directions.
Furthermore, repositioning of the feed while correcting the error in one plane further distorts the error in the other plane. This error is characteristic of most three dimensional bootlace lenses for beams not coinciding with perfect foci on the lens axis.